Abert’s squirrels a year-round symbiotic forest resident

Courtesy of National Park Service

This fall, while leading a group of third-graders around Junction Creek Campground near Durango, we were enthralled with the bustling activities of Abertís squirrels getting ready for winter. These small, furry, tree-dwelling animals are fun to learn about and observe.

Abertís squirrels can be grey, reddish or black on the back with a white belly. They weigh about 2 pounds, and their head and body are about 12 inches long with a 9-inch tail. Their signature ear tufts are longer in winter and may disappear in summer.

Abertís squirrels are found in Colorado, southeast Utah and south into New Mexico, Arizona and Mexico. There are nine subspecies of Abertís squirrels that are associated with different areas.

They also are directly associated with ponderosa pines. These trees provide them with just about everything they need for food and shelter. They build nests out of small branches that they can weave into a safe place to raise young.

Unlike most squirrels, Abertís squirrels are active year-round. They mate in the spring, anywhere from February to early June, and have a litter of three to four pups about 40 days later. Abertís squirrels do not hibernate or cache their food for the winter (except for those mushrooms), so evidence of them can be found throughout the season.

On simple and complicated levels, Abertís squirrels are connected with their ecosystem. They eat seeds from pine cones, inner bark, buds and pollen cones. They will pluck a mature pine cone from a tree, remove the scales, then eat the protein-packed seeds. They eat pine needles and the inner bark of twigs, the latter especially in winter.

Evidence of Abertís squirrels include seeing the ends of ponderosa pine branches on the ground; this happens when they are getting to that inner bark. They gnaw off the ends of the branches, letting the pine needle ends fall to the ground. They retain the remainder of the branch, removing the outer bark and eat the inner bark, or phloem. They then drop the remainder of the twig on the ground.

In summer, the squirrels will collect mushrooms and hang them all over trees to dry. These become winter meals.

When Abertís squirrels eat some buds and shoots of the ponderosa pine, the uneaten part of the twig falls to the ground. This provides food for mule deer.

Goshawks are the main predator of Abertís squirrels. Other predators include bobcats, mountain lions and coyotes.

Abertís squirrels return to the same ponderosa pines year after year. Some researchers theorize the squirrels prefer trees that have more sugary sap as well as more carbohydrates, nitrogen and sodium, and less iron, mercury and some monoterpenes (toxins).

The squirrels actually help the trees, in addition to using them.

Ponderosa pines have a mutualistic relationship with mycorrhizal fungi, found in the soil. The fungi extends the reach of the treesí roots, helping them get to more water, phosphate, nitrogen and other nutrients, and the trees provide the fungi with carbohydrates.

From late spring to early fall, the squirrels eat the Ďfruiting bodyí of the ectomycorrhizal fungi Ė the part that shows up above ground for part of the year. This mushroom has spores throughout, which the fungi use to reproduce. When eaten, the spores survive through the digestive tract of the squirrel, coming out in its scat, and thus are spread throughout the ponderosa pine forest, especially those trees where the squirrels like to hang out. This is an amazing trifecta of a mutualistic relationship Ė among the Abertís squirrels, ponderosa pines and ectomycorrhizal fungi.

This winter, when you are out exploring the beautiful ponderosa forests, take a quiet moment to look around Ė chances are youíll find evidence of some of our tree-dwelling, and tree-helping, friends around.

Gabi Morey is education outreach director with San Juan Mountains Association. SJMA is a nonprofit dedicated to public land stewardship and education.